Stably transfected HEK293 cells expressing wt, mutated, or ΔE9-PS1 were incubated for 24 hr with 25 µM of Z-IE(Ot-Bu)A-Leucinal. Cells were then lysed and immunoreactivities of HMW-PS1 and LMW-PS1 were revealed with αPS1Loop antibody and quantified as described in Materials and Methods. Values are expressed as the percent of control corresponding to immunoreactivity observed in identical cells in the absence of inhibitor and are the means ± S.E.M. of the indicated number of independent experiments, nd, not detectable.